This invention relates to a process for the polymerization of olefinic compounds, especially a process in which a olefinic compounds are separated and recovered for use from mixtures containing them downsteam of the polymerization reactor. This invention also relates to a process for the improved utilization of olefinic compounds.
As is well known, the polymerization of olefinic compounds is a commercially important process in view of the multitude of uses of polymers and copolymers. Commonly the olefinic compound or mixture of olefinic compounds to be polymerized is passed into a reaction zone where polymerization takes place. Generally a catalyst is used to promote such polymerization. Polymerization can be conducted by numerous processes, such as by solution, slurry or gas phase processes.
Feed olefin to polymerization processes commonly contains impurities such as saturated materials having approximately the same molecular weight as the olefin. For example, ethylene commonly contains a small amount of ethane and in some cases smaller amounts of methane and propane. Propylene commonly contains small amounts of propane and to a lesser degree ethane and butane. Additional saturated materials can be formed in the reaction zone by reaction of the hydrogen which was added to control molecular weight of the polymers, with olefin feed to the reactor. The olefin is hydrogenated and becomes saturated, thereby forming the corresponding saturated compound of the olefin. Also, low molecular weight polymers formed during polymerization may or may not be saturated. Therefore, it can be seen that there are numerous sources of saturated material in the polymerization process. This saturated materials generally do not take part in the polymerization process but merely pass through the process unreacted. While in many cases these saturated materials are not detrimental to the polymerization reaction itself, reactor capacity can unnecessarily be used in passing through these inerts. These saturated materials are removed with final product and unreacted olefinic compound from the reaction zone. In most processes, the unreacted olefinic compound and saturated contaminant is recycled to the reactor. To prevent concentration of the contaminant in the reactor, a portion of this recycle stream is sent to disposal such as flare.
If an attempt is made to recycle the unreacted olefin, the saturated materials contaminant will also be recycled and will build up in the reactor. This build up will lead to a decreased throughput of the polymerization process and eventually an uneconomical process. It is commonly economically unattractive to remove the saturated material contaminant from unreacted olefinic compound through distillation techniques because of the similarity in boiling points between these materials and the expense of distillation equipment required. Often small streams of unreacted olefinic compound and contaminants are sent to disposal such as flare rather than attempt to recover the unreacted olefin. This is obviously a waste of raw materials.
As previously mentioned saturated hydrocarbons are sometimes passed into the polymerization zone so as to help introduce the catalyst via a catalyst/saturated hydrocarbon slurry, or in order to control the reaction temperature. These saturated hydrocarbons generally do not chemically take part in the polymerization reaction and are passed out of the reaction zone along with the formed polymer and unreacted olefin. The mixture of materials removed from the polymerization reactor commonly comprises polymer catalyst residue, unreacted olefin, various saturated materials including hydrocarbon, and small amounts of numerous other impurities. While it is desirable to recover the unreacted olefin for recycle thereby improving overall process efficiency, it is commonly not done or carried out only to a limited extent because of the difficulty of separating the saturated from unsaturated materials. Commonly a stream containing unreacted olefinic compound and saturated material is sent to flare.
A most common technique of separating materials is by distillation. However, where the materials have similar boiling points, such as an olefin and its corresponding saturated compound, suitable separation by distillation is expensive. Another method of separating olefinic compounds from saturated materials is through the use of semipermeable membrane separators. The mixture of compounds to be separated contacts one side of a semipermeable membrane and an enriched olefinic stream is removed from the other side. While polymeric membranes can be made which are capable of such separation, the preferred separators use an aqueous liquid barrier containing metal containing ions in conjunction with semipermeable membranes. This type of separator and process is more fully described in U.S. Pat. Nos. 3,758,603; 3,758,605; 3,770,842; 3,812,651; 3,844,735; 3,864,418; 4,014,665; and 4,060,566 which are hereby incorporated by reference and made a part hereof. U.S. Pat. No. 4,060,566 discloses that the sweep fluid such as butane, hexane or others may be used to remove olefins from the vicinity of the membrane may also serve in downstream processing of the separated material, either as a reactant, or as a solvent.
It is an object of this invention to provide an economically improved polymerization process which improves the utilization of olefinic compounds.
It is an object of this invention to reduce the amount of raw material wastage, especially olefinically unsaturated monomer, in a polymerization process.
It is further an object of this invention to provide an efficient method for the recovery of unreacted olefins from mixtures from polymerization processes.